Differential amplifiers function, in general, to amplify the difference between two signals. They are used in a wide variety of measurement applications where response from DC to many megahertz is required. In addition, differential amplifiers are the basic stage of integrated operational amplifiers with differential inputs.
While there are a wide variety of differential amplifier circuit topologies, most, if not all, include a constant current source, two load resistors and symmetrical transistors input stages for splitting the constant current between the load resistors. Usually, the output voltage is taken as the difference in the voltage drops across the load resistors; in this manner, large variations in output voltages may be achieved with extremely small input voltage differentials. One such amplifier, and perhaps the simplest, is the symmetrical emitter-coupled differential amplifier. This amplifier topology comprises two transistors having their emitters connected to a current source, their collectors each tied to a positive potential, possibly through an equivalent load resistance, with the output taken in the form of a current from the collectors which is related to the input potential difference between the two bases. The collector current is most often converted to a voltage by the load resistance. The circuit makes an excellent differential amplifier if the emitter resistance is kept relatively large.
For all differential amplifiers there are associated therewith certain differential voltage gain and input resistance performance characteristics. These characteristics and others determine the usefulness of the amplifier in any given application. For example, some applications may require an amplifier with a high input impedance and moderate gain, and vice versa. For the simple emitter-coupled differential amplifier, differential voltage gain is directly proportional to the level of the constant current (I.sub.o), and the load resistance (R.sub.1); differential input resistance is directly proportional to the input transistor .beta. and inversely proportional to I.sub.o. Therefore, in order to increase the gain without changing R.sub.1 and thereby degrading the output impedance, I.sub.o must be increased, giving rise to an undesirable decrease in input resistance.
One well-known modification to the basic differential amplifier that provides for increased input resistance is the addition of a single transistor to each side to form a Darlington pair. This Darlington differential amplifier topology provides a differential input resistance which, compared to differential input resistance of the simple emitter-coupled type of amplifier, is a factor of .beta. larger. As a result, differential gain may be improved, by increasing I.sub.o, while maintaining a relatively high differential input resistance.
While the two foregoing topologies provide adequate performance for most applications, improvements in the differential gain and input impedance characteristics of differential amplifiers have been a constant goal for designers. Accordingly, there exist numerous differential amplifier designs, each attempting to maximize certain performance characteristics. However, until now there have been no significant improvements in differential amplifier performance without relatively large increases in their corresponding degree of complexity, usually requiring multiple stages with increased power dissipation. As a result these improvements have had limited usefulness, especially in designs where power consumption and space limitations are critical.